1. Field of the Invention
The field of art to which this invention pertains is aromatic polyamide fibers and, more particularly, it is directed to methods of dyeing these fibers.
Specifically, the instant invention is a method of dyeing a fiber structure or two of dried or crystalline poly(m-phenylene isophthalamide) fibers or filaments with a water-soluble dye padded onto the fibers, along with a small amount of a suitable dye carrier. The tow is heated with steam at a temperature of at least about 120.degree. C. for a time sufficient to dye the fibers adjacent their surface and at about 165.degree. C. to completely due the fibers throughout their structure.
By following the method of this invention, amorphous or crystalline poly(m-phenylene isophthalamide) fibers may be efficiently dyed a bright deep color in a short period of time with highly compatible water-soluble dyes, at relatively low temperatures, e.g., below about 165.degree. C., using carrier amounts of less than 5 wt. % based on fiber weight, and preferably even lesser amounts (1 to 2%) without loss of dye effectiveness.
2. Description of the Related Art
Aromatic polyamide fibers are known to the art. They have outstanding properties such as high tensile strength, flame and heat resistance, and good flex life which make them suited to be formed into fabrics usable as protective clothing, and for many other uses.
More specifically, this invention is directed to a method of dyeing aromatic polyamide fibers of a poly(m-phenylene isophthalamide) polymer, hereinafter referred to as "MPD-I fibers". Such fibers, which are described in greater detail in U.S. Pat. No. 3,287,324 to Sweeny, for example, posses many useful properties, as indicated above.
Briefly described, the MPD-I fibers of the type involved in this invention are typically formed by extruding a spinning solution through orifices in a spinneret. Such fibers may be dry-spun or wet-spun, by known methods, to form a "water-swollen" fiber structure. Such fibers, which are substantially amorphous at this stage, in turn, are brought into contact with an aqueous extraction bath, then combined in "water-swollen" form into tow and appropriately dried, generally at about 140.degree. C.
For certain uses, these dried amorphous fibers may be further heated to form crystalline MPD-I fibers. In either case, however, it is well known to the art that dried MPD-I fibers, whether amorphous or crystalline are very difficult to dye.
As a result, many methods have evolved, over the years, for dyeing MPD-I fibers in various forms. U.S. Pat. No. 4,883,496 to Ghorashi describes a number of these methods. The teachings of this patent are incorporated herein by reference.
As described in this patent, the most generally accepted method for dyeing MPD-I fibers has been to dye the fibers, after they have been dried, in a pressure vessel at 121.degree. to 132.degree. C., using an aqueous dyebath containing a large percentage of a carrier, such as acetophenone. For amorphous fibers, the amount of carrier used is around 40%, based on the weight of the fibers; for crystalline MPD-I fibers, the amount of carrier used is even higher, sometimes approaching 100%, based on the weight of the fibers.
This method has proven to be very effective for dyeing dried MPD-I fibers with water-soluble dyes. However, the large amount of carrier involved can present certain cost and dyebath disposal problems to the user. Further, several hours are generally required to achieve the depth of color desired in the finished product. Also, this method is more suitable for dyeing fabric, than for dyeing tow.
Accordingly, a method has long been sought for dyeing amorphous or crystalline MPD-I tow using water-soluble dyes, without the need to use aqueous dyebath techniques, to obtain a wide range of colors while retaining good fiber properties. It has been especially desired to achieve a process for applying such dyes at relatively low temperatures, e.g., 165.degree. C. or less, since many otherwise desirable dyes are unstable at higher temperatures. And, it further has been desired to be able to dye crystalline or amorphous MPD-I tow continuously within a relatively short time, e.g., 30 minutes or less. It also has been desired to be able to dye MPD-I tow with low levels of dye carriers, for example, less than 40% by weight of fibers.
This invention solves these and other problems found in the prior art by surprisingly finding that by heating dried, crystalline or amorphous MPD-I fibers with steam, heated within certain temperature ranges in the presence of a very small amount of carrier, it is possible effectively to dye the fibers. Specifically, it has been found that such fibers may be dyed with a water-soluble dye which is padded into the fibers, along with from about 0.5 to 5 wt. % of a suitable carrier, and then heated with steam at temperatures from 120.degree. to 165.degree. C. At the lower 120.degree. C. temperature range, the fibers are effectively surface dyes; for more complete dyeing of the fiber structure the higher 165.degree. C. temperature range is required. These fibers may be dyed in a very short period of time (e.g., less than 30 minutes), with little or no residual carrier disposal problem. In so doing, the method of this invention provides the art with an effective, improved means of dyeing amorphous or crystalline MPD-I fibers with a large variety of water-soluble dyestuff.